Total hip arthroplasty (THA) is the most effective intervention for end-stage degenerative conditions of the hip such as arthritis. However, improvement in technologies and surgical techniques has not seen a corresponding decrease in premature failure and revision requirement. Orientation of the implant should be defined in the local pelvic coordinate system, solely established by the anterior frontal pelvic plane (AFPP). Current X-ray fails to incorporate the AFPP orientation. Intra-operative measurement of the AFPP relies on percutaneous palpating bony landmarks which are hardly reachable in obese individuals. The missing or misidentified AFPP will introduce significant errors. This proposal proposes a new methodology to acquire the AFPP from plain X-rays and a novel ultrasound probe to correct the soft tissue errors intraoperatively in THA. It is expected that these technologies help to improve the outcomes of the THA. Specific Aim 1: To develop a new method to measure AFPP from 2 plain x-ray images from different angles, including a standardized protocol for taking plain X-ray images from two accurately defined views, and an associated algorithm to measure AFPP from these plain X-ray images. The proposed methodology will be validated using a 3D pelvic model reconstructed from CT scans. When the AFPP is obtained from the plain x-rays, the influence of the AFPP variation on the interpretation of the AC orientation will also be evaluated. In addition, the variance of pelvic position will be evaluated in a small patient group. It is expected that this simple yet precise methodology can be an accurate and cost effective substitute for CT scan in determining AC orientation for post-THA followup. Specific Aim 2: To develop an innovative palpating probe with ultrasonic transducer for Computer Assisted THA to precisely measure the thickness of soft-tissue overlaying each bony landmark intraoperatively. To validate the proposed methodology through cadaver experiment and calibrate the proposed ultrasound probe on human subjects. The developed technologies will be validated on human subjects and cadavers through methodologies established in our preliminary studies.